1. Field of the Disclosure
The present invention relates generally to power converter controllers, and more specifically, the invention relates to sampling circuits for power converter controllers.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. utilize power to operate. Because power is generally delivered through a wall socket as high voltage alternating current (ac), a device, typically referred to as a power converter can be utilized to transform the high voltage alternating current (ac) input to a well regulated direct current (dc) output through an energy transfer element. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. In operation, a switch is utilized to provide the desired output quantity by varying the duty cycle (typically the ratio of the on-time of the switch to the total switching period), varying the switching frequency or varying the number of pulses per unit time of the switch in a power converter.
A power converter may use a controller to provide output regulation to an electrical device (generally referred to as a load) by sensing and controlling the output of the power converter in a closed loop. More specifically, the controller may be coupled to a sensor that provides feedback information about the output of the power converter in order to regulate the output quantity delivered to the load. The controller regulates the output quantity delivered to the load by controlling a switch to turn on and off in response to the feedback information from the sensor to transfer energy pulses to the power converter output from a source of input power such as a power line.
The sensor used in the power converter to provide the feedback information may include an optocoupler that receives information about the output voltage directly from the output of the power converter. The output of the power converter is also coupled to a secondary winding of the energy transfer element. This type of control scheme is typically referred to as “secondary-side control.” Another type of control scheme, commonly referred to as “primary-side control,” may alternatively be utilized by the controller. In primary-side control, the sensor may include a primary-referenced winding (e.g., a bias winding) of the energy transfer element to provide a signal representative of the output voltage of the power converter immediately after a switching event that delivers energy to the output. Although primary-side control eliminates the cost and the power consumed by an optocoupler, the output voltage cannot be sensed in the absence of switching. In addition, there is a limited amount of time that the signal provided by the primary-referenced winding is representative of the output voltage, especially during light-load conditions.